The present invention relates to a method of purifying brine to be subjected to electrolyzation for producing caustic soda and chlorine, and a system for carrying out the same.
In electrolyzing salt, it is usual practice to supply brine prepared by dissolving salt in water and purified by removing impurities contained in the salt solution, to an electrolyzer. Recently, the electrolytic process for electrolyzing brine has been converted progressively from the membrane system into the ion exchange membrane system with the increase in demand for electrolytic caustic soda of higher purity; consequently, the purification of brine has become regarded as important.
When brine containing much hardness components is supplied to the electrolytic process of the ion exchange membrane system, the performance of the ion exchange membranes is deteriorated and the life of the same is remarkably reduced. Accordingly, the brine needs to be treated through filtering layers packed with a chelate resin, for high purification before the brine is supplied to an electrolyzer.
FIG. 1 illustrates a conventional brine purifying system. Referring to FIG. 1, dechlorinated dilute brine recovered from an electrolyzer 14 is returned from a dechlorinated dilute brine tank 13 to a saturated brine tank 1. The saturated brine tank 1 is replenished with salt and, if necessary, water to prepare saturated brine. The saturated brine is supplied to a mixing tank 2, where caustic soda or sodium carbonate is mixed in the saturated brine to produce so-called brine mud. The saturated brine containing the brine mud is supplied to a thickener 3, where the most part of the brine mud is deposited and the deposited brine mud is discharged outside the system from the bottom of the thickner 3.
Since the supernatant saturated brine supplied from the thickner 3 usually contains a minute amount of unsoluble solid matters, the supernatant saturated brine is stored temporarily in a supernatant saturated brine tank 4, and then the supernatant saturated brine is supplied to a filtering apparatus 6. The filtered and purified brine is stored in a purified brine tank 7, and then the purified brine is supplied to a chelate resin tower 8, where a small amount of the residual impurity ions are removed. Then, hydrochloric acid is added to the purified brine in a pH adjusting tank 9 to adjust the pH of the purified brine. Then, this high purity brine is supplied to the electrolyzer.
However, it was found that the life of the chelate resin is greatly dependent on the amount of suspended solid matters remaining in the brine and that a high degree of separation of the solid matters is necessary in addition to solid matter separation by means of the thickener. In order to meet such a requirement, purified brine obtained by separating the solid matters from the saturated brine by means of the thickner. In order to achieve the satisfactory filtration of the supernatant brine through the perfect separation of the solid matters, the brine feed rate needs to be reduced and the filtering apparatus needs to be washed frequently for regeneration, which causes an increase in the brine purifying cost.